1. Field of the Invention
The invention pertains generally to the art of detector circuits and specifically to those circuits which are used to detect input signals having amplitudes which are within the same range as the amplitude of the noise present within the useful frequency band of the detector.
2. Description of the Prior Art
A constant false alarm rate detector was disclosed by A. M. Nicolson and R. J. Brophy in the U.S. Pat. No. 3,755,969 entitled "Detector Having A Constant False Alarm Rate and Method For Providing Same, " issued Aug. 28, 1973 and assigned to the same assignee as the subject invention. As disclosed therein, a detector subject to temperature variations and power supply drift is included in an avalanche transistor circuit having a variable threshold sensitive to input signals within a useful frequency band and noise which produced threshold signals when the amplitude of the input signals or the noise exceeded the instantaneous value of the variable threshold. The rate at which the threshold signals were produced was determined in an N-bit storage device coupled to the avalanche transistor circuit. The storage device was coupled to an N-input summation circuit which provided a feedback signal in the form of a variable amplitude output signal in which the amplitude varied at a rate proportional to the rate at which threshold signals were produced by the avalanche transistor. The variable amplitude signal was applied to a series circuit which shunted the collector current of the avalanche transistor circuit at a rate commensurate with the rate that the variable amplitude signal varied in response to noise but at a substantially lower rate than the rate the variable amplitude signal varied in response to input signals having an amplitude greater than the instantaneous value of the avalanche transistor variable threshold. As further disclosed therein, a gate circuit produced a pulse which was coupled into the emitter of the avalanche transistor to control the period of time during which breakdown of the avalance transistor could occur in response to noise or input signals having amplitudes greater than the threshold level of the avalance transistor.
As generally known to those skilled in the art, avalanche transistors have to be chosen by the manufacturers from among a set of given transistors and although the yield can often be high, manufacturers do not supply avalanche transistors with specified performance in the avalanche region. Further, the power supply voltage used with avalanche transistors is not compatible with the 5 volt power sources used with conventional digital logic elements. A tunnel diode, on the other hand, has the advantage of being a device which is commercially available with specified parameters. Thus the improved detector disclosed herein does not require circuit adjustments to compensate for different parameters in the variable threshold device, because the parameters in the tunnel diodes are specified and it is merely a matter of direct replacement to substitute one tunnel diode having the same parameters for another. Whereas with avalanche transistors the performance in the avalanche region is not specified. Therefore it requires additional testing and circuit adjustments to insure similar operation after substituting one avalanche for another.
Furthermore, the control circuit described herein enables the time interval during which the tunnel diode detection device is sensitive to be controlled with nanosecond precision.